In order for a motor to generate a suitable assist steering power according to a steering operation (steering) of a driver, a control device (an ECU) computes the assist steering power based on various inputs applied to a steering shaft by the steering operation of the driver, such as a steering torque and a vehicle speed, and drives the motor based on the computed assist steering power.
The conventional electric power-steering system basically controls the motor so that the assist steering power applied by the motor becomes larger as the steering torque becomes larger.
When the driver turns a steering wheel, the assist steering power is generated according to the applied steering torque, and so the steering operation becomes lighter.
However, it is difficult to give the driver a correct impression via the steering wheel of the reaction between the road and wheels, only by determining the assist steering power based on the steering torque.
In other words, it is difficult to configure a target steering torque according to the road surface force and generate the assist steering force according to the target steering torque (that is, realizing the target steering torque according to the road surface force) in order to impress on the driver the steering operational feeling according to the road surface force.
On the other hand, a technology that controls a motor based on a deviation of a target steering torque from an actual steering torque is disclosed in Japanese Patent Application Laid-Open Publication No. 2004-203089.
With the technology disclosed in the publication '089, a force (namely, a road surface reaction) given to a tire is acquired by requesting an output torque from a sum of an actual steering torque (torque given by the driver) and an assist torque (torque given by the motor).
Then, the target steering torque is configured according to the obtained output torque (the road surface reaction force).
Thereby, the target steering torque can be determined unambiguously.
Therefore, the target steering torque whose change (namely, change of the road surface reaction force) is compensated for by the output side torque can be configured by considering influences of friction and inertia superimposed on the steering torque when turning sharply the steering wheel, for example, or by modifying based on a vehicle movement state.
However, although the target steering torque is configured considering the road surface reaction in the technology disclosed in the publication '089, the assist torque is generated based on a the torque applied to the steering wheel called steering torque fundamentally.
Therefore, although the required torque on the shaft can be suppressed (that is, the steering operation can be made light), the convergence of the vehicle cannot be secured because, a revolving speed of the motor (revolving speed of the steering shaft) cannot be controlled appropriately in a situation where the twist decreases when returning the steering wheel to a neutral position, for example, or under a situation where the steering wheel reverts to the neutral position with few twists when the hands of the driver separate from the steering wheel after steering.
For example, although it is satisfactory in particular when turning the steering wheel sharply, rotation of the shaft in a direction returning to the neutral position is speeded up because the twist of the shaft returns when steering wheel return and the assist torque also decreases by this, therefore, the stability and the convergence of the vehicle is spoiled.
Although it is good for the driver when operating the steering wheel firmly, a motion of the vehicle becomes sensitive and hard to converge to the desired direction when power applied to the steering wheel is reduced or the hands are separated from the wheel, thus gives a feeling of insecurity to the driver as the vehicle speed gets faster.
Hence, securing the operation stability of the whole vehicle appropriately (realizing the suitable vehicle movement characteristics) is not made with the technology disclosed in the publication '089.
Therefore, the driver must respond to a vibration force that is transmitted to the steering wheel when a movement of the vehicle is unstable, and there is a possibility of interfering with the driving operation.
If damping is used in order to secure convergence, resistance is given when returning the steering wheel and convergence improves, however, resistance is also given when turning the steering wheel deeply, and a steering feeling to the driver is spoiled.
If control actions are changed for the time of turning and the time of returning the steering wheel, respectively, in order to avoid the problem mentioned above, the control processing may become very complicated and conformity between them both may become difficult.
On the other hand, as one of the technologies for securing operation stability appropriately, and as a technology for reducing a sudden feeling of return when returning the steering wheel and raising the convergence of the vehicle direction without spoiling the feeling when turning the steering wheel, a convergence control technology that calculates a fundamental assist steering power (base assist torque) based on the steering torque and the speed of the vehicle, computes a modifying torque for modifying the base assist torque based on the steering torque and the motor speed (rotation angle speed), and modifies the base assist torque by the modifying torque is known, for example, (refer to Japanese Patent Application Laid-Open Publication No. 2010-264913, for example).
That is, while the steering torque is a physical value in which the operation state of the steering wheel by the driver is reflected, the motor speed is a physical value in which the influence of the road surface reaction is also reflected in addition to the operation of turning the steering wheel by the driver.
Therefore, based on the steering torque and the motor speed, a control mechanism can be constituted that can realize the convergence control such that the operational feeling is not spoiled when turning the steering wheel, and the sudden feeling of return of the when to the neutral position by the road surface reaction is reduced when returning the steering wheel.
Therefore, in the publication '913, based on the steering torque and the motor speed, suitable stability (suitable vehicle movement characteristics) of the whole vehicles is realized by preventing the steering feeling of the driver from being spoiled when turning and returning the steering wheel, by generating the assist compensation quantity differently when turning the steering wheel and returning the steering wheel different, and modifying the base assist torque by the assist compensation quantity.
Various technologies for securing operation stability appropriately are proposed, and a torque modifying technology for aiming for a reduction of frequent correction operation and operating duty mitigation by realizing vehicle movement suitable for the intention of the driver is disclosed in Japanese Patent Application Laid-Open Publication No. 2007-22373, for example.
With the technology, a vehicle movement state is estimated based on a sum of the base assist torque and the steering torque, and a quantity of torque modifying the base assist torque is generated so that the characteristics become the desired ones.
Then, in order to reconcile a realization of the characteristics of the steering reaction according to the road surface reaction and a realization of the suitable operation stability (suitable vehicle movement characteristics) as the whole vehicles, it is considered to try the combination of the technologies disclosed in the above-mentioned publications '089 and '913, for example.
Specifically, a control mechanism as shown in FIG. 14 can be built.
In the control mechanism shown in FIG. 14, a current feedback (FB) section 142, gives an assist steering force according to an assist torque command Ta to a steering shaft by driving a motor 110 by applying a drive voltage Vd according to an assist torque command Ta inputted to the motor 110.
In a controlled object (steering system mechanism) 100 from the steering shaft to wheels, a motor speed ω that is a revolving speed of the motor 110 and a steering torque Ts are detected.
The current FB section 142 detects a current that flows into the motor 110 (motor current Im) and performs a current feedback (FB) control so that the value of the current becomes a target current (value matching the assist torque command Ta).
Then, based on the detected motor current Im and the steering torque Ts, a force estimator 121 estimates a road surface force (road surface reaction), and generates an estimated force Tx as an estimated value.
Then, a base assist section 120 generates a base assist command Tb* based on the estimated force Tx.
The base assist section 120 specifically has a target generating section 122 that generates a target steering torque Ts* based on the estimated force Tx, a deviation computer 123 that computes the difference (torque deviation) between the steering torque Ts and the target steering torque Ts*, and a controller 124 that generates a base assist command Tb* for controlling the motor 110 (that is, a torque feedback control is performed) based on the torque deviation so that the steering torque Ts matches the target steering torque Ts*.
Thus, by controlling the motor 110 by the obtained base assist command Tb*, realization of the characteristics of the steering reaction according to the road surface reaction becomes possible.
That is, the target generating section 122 configures the target steering torque Ts* according to the road surface force relative to the road surface reaction that the force estimator 121 estimated.
Then, the driver can feel the response according to the road surface force because the controller 124 performs the torque feedback control so that the actual steering torque Ts becomes the target steering torque Ts*.
On the other hand, a modifying section 130 has a torque modifying section 131 that generates a torque modifying command Tr based on the steering torque Ts and the motor speed ω, and realizes the convergence control.
A specific composition and function of the torque modifying section 131 are the same as that of a controller disclosed in the publication '913 mentioned above.
Therefore, by modifying the base assist command Tb* by the torque modifying command Tr obtained in the modifying section 130 (here, both are added with an adder 141), and if the modifying result as the assist torque command Ta is given the current FB section 142, realizing the suitable operation stability (suitable vehicle movement characteristics) as the whole vehicle becomes possible for the time being.
However, at least one big problem exists in the control mechanism shown in FIG. 14.
The problem is that the base assist section 120 cancels the modifying operation by the modifying section 130.
That is, in the control mechanism shown in FIG. 14, force estimation with the force estimator 121 is performed based on the motor current Im finally supplied to the motor.
The motor current Im responds to the assist torque command Ta that is modified by the torque modifying command Tr that the modifying section 130 generated to the base assist command Tb* that the base assist section 120 generated.
That is, the force estimator 121 estimates the road surface force using the motor current Im as a result modified by the modifying section 130.
In other words, when steering a steering wheel sharply, for example, the base assist command Tb* is modified so that the assist steering power decreases by the convergence control in the modifying section 130.
Then, the motor current Im falls for the amount of the modification, and thereby, the force estimated value (estimated force Tx) by the force estimator 121 also falls.
Then if the estimated force Tx falls, the target steering torque Ts* also falls, and by this, the controls base assist command Tb* increases, i.e., the assist steering power is controlled in a tendency of increasing so that the steering torque becomes small.
That is, even though the torque modifying command Tr is generated convergence control by the modifying section 130 in order to reduce the assist steering power, the assist steering power is controlled conversely in a tendency of increasing by the base assist section 120.
As a result, the convergence control by the modifying section 130 is not reflected (convergence control is canceled by the base assist section 120).
Thus, with the control composition shown in FIG. 14, it is incompatible in a realization of the characteristics of the steering reaction according to the road surface reaction, and in a realization of the suitable operation stability (suitable vehicle movement characteristics) as the whole vehicles.
Therefore, it is difficult to raise a running performance of the vehicle, i.e., it is difficult to enable the driver to drive the vehicle intuitively, and feels reliable to operate the vehicle.